MacOS – Upgrade os snow leopard to mountain lion without time machine

This isn't something that Apple tests well (or perhaps at all) - so you might have some crashes, but in general these are minor or confined to one app.

If you do the downgrade, just be systematic about watching for crashes, keep all the backups. You can always trash the preferences if you have odd feelings or just trash them all after the restore to start "clean" with Snow Leopard defaults but all of your Lion data.

It would be much better to export the data from Lion and restore from your last Snow Leopard backup, but your time may be spent there for no need or you could be "committing" your time later to debugging things.

It really depends on how much changes you made and the details of what changed file by file, preference by preference. Some will not notice a thing, you'll want a little luck on your side if you try it, but perhaps not even need it.

Let us know how it goes!

In general, there are no issues or side-effects of using a drive connected in turn to any number of macs with mixes of OS levels since each Mac only writes to its own backup folder.

The obvious exception is Lion style FileVault encryption where Snow Leopard and older macs will be prevented from reading or writing the drive.

The only other problem is if you mix lion and snow leopard on the same Mac and (or) let lion "inherit" the backup of another machine.

In that one case, the contents of the backup are mixed and you can't be guaranteed a restore of system files will work. Your data and pictures and other files will be fine but anything tied to database files or a specific version of the mail app (for example) can depend on versioning.

This isn't really a side effect of Time Machnie changing how it works, but of the OS and app changes.